This invention relates to the field of AM stereophonic receivers and, more particularly, to the prevention of intermodulation distortion and other forms of distortion due to the presence of the pilot tone in the stereo difference signal.
In stereophonic broadcasting, a "pilot tone" is usually added to the modulating signal in such a way that a receiver can detect the presence of the pilot tone and use that information to indicate to the user that a stereo station has been tuned in. Typically, the information is also used for other purposes such as enabling stereo operation in the receiver.
In AM stereo transmission, a subaudible or nearly subaudible tone is usually added to the L-R or difference channel before the difference signal is modulated onto the carrier. In the receiver this tone is separated from the L-R signal and utilized as desired. However, due to the fact that the pilot tone is so near in frequency to the desired audio signals, it is sometimes difficult to completely eliminate the tone from the L-R signal.
The pilot tone can have two undesired effects, both appearing in the audio circuits where it can produce objectionable sounds in the audio output channels and can also cause apparent shifting of the audio "image". The latter effect is a form of intermodulation distortion and is caused by the pilot tone affecting the phase locked loop used in the detection circuits of some AM stereo receivers.
Additional information on such receivers may be found in U.S. Pat. Nos. 4,159,398, 4,405,837 and 4,410,762 assigned to the assignee of the present invention.